A systematic investigation of the factors influencing the reduction of iron(III) and iron(II) porphyrins is being undertaken. Changes in redox mechanism as a function of porphyrin ring moiety, solvent media, and counterion are being determined. Iron(III) porphyrins have been prepared with eight different anions ranging from the non-coordinating ClO4 minus anion to the tightly held F minus anion. A 700 mV stabilization of the positively charged iron center is observed as the coordinating ability of the counterion increased. The degree of axial complexation to the iron center by sterically hindered and unhindered nitrogenous bases is being investigated as a function of the associated ion pair. The extent of ion pair coordination is found to directly influence not only the strength of the adduct formed, but also the mechanism of electroreduction. Rates of heterogeneous electron transfer will be determined for each of the electron transfer processes, including the binuclear oxo- and nitrido bridged porphyrins. Half wave potential and ligand addition measurements will be made over a wide range of temperatures in order to determine the relevant thermodynamic parameters. Comparisons will be made between the electron transfer reaction entropy and the thermodynamics of ligand addition.